The present invention is directed to a method and a device for clinching metal sheets disposed one over the other in planar fashion, where a joining tool, composed of a punch and a die having an anvil, partially displaces metal sheets, profiles, or multi-sheet connections, out of the sheet-metal plane, under the action of forming energy, compressing the same.
In known clinching processes, during a working stroke, a punch penetrates into the metal sheets to be joined, a one-piece or segmented die producing the form on the opposite side.
The forming of the indentation by the tools is not only accompanied by force-locking, but also by the known form-locking effect, the so-called undercutting, between the joint partners, the process usually being carried out until a targeted reduction in the bottom thickness is reached. In the process, in the punch-side sheet, the original material thickness is substantially thinned to the so-called throat thickness in the area of the clinching punch. In principle, to increase the strength of the connection, a high undercut and a substantial throat thickness are essential. The strength of the connection is further enhanced by cold-pressure welding in the zones of greatest reshaping between the joint partners.
A joining device is known from European Patent Application No. 0 890 397 A1. The percussive or pulsating driving of the punch into the punch-side part to be joined produces so great a punch acceleration that, as the punch penetrates into the part to be joined, it draws in less material from the part to be joined into the die than in previous, standard joining operations employing the hydraulic application of force. When working with the pulsed application of force, the area of the part to be joined underneath the end face of the punch is stretched more vigorously.
For the application of force, various drives can be accommodated in the percussion mechanism provided for penetration of the punch. Useful in this context are pneumatic drives, such as those in a pneumatic chisel or pneumatic hammer, electric drives, for example having a rotary crank drive, unbalance motors, or electromagnetic, electropneumatic, or servohydraulic drives. These enable the frequency, the impact strength and impact speed to be varied within a certain range.
German Patent Application No. DE 100 60 035 A1 discloses a device for mechanically joining metal sheets, profiles, and/or multi-sheet connections, where the punch of a joining tool composed of an upper part and die, penetrates into a part to be joined by the action of a plurality of rapidly successive impacts, or by pulsating at an excitation frequency.
The surface area of the punch is designed to inhibit slip, at least in the area of its front, unattached end section. The result is an increased coefficient of friction between the surface of the punch tip and the part to be joined.
The stretching of the material located underneath the end face of the punch tip is further reduced by an additional slip-inhibiting design of the end face of the punch surface. This inhibition of slip can be accomplished by roughening the punch surfaces mentioned above. It is customary for the surface area of the unattached end section and the end face of the punch to be provided with circumferential grooves or ribs. In another form of inhibiting slip of the punch surfaces, the surfaces can be provided with a slip-inhibiting coating.
The increase in the coefficient of friction of the punch surfaces leads to more material being drawn in from the area of the part to be joined bordering on the joint when the punch penetrates into the part to be joined, this material being available for the undercut-forming upsetting or back-taper process, and the greater stretching associated therewith in the throat region of the joint leads to a greater strengthening of this throat region and to improved material-strength characteristics of the joint. Via the degree of forming and the undercut, the strength of the connection can be further increased here as well by improving the cold-pressure welding.